The present embodiments relate to phase change ink compositions that can be used for ink jet printing in a variety of applications. A relatively new type of phase change ink is an ultraviolet (UV) curable solid ink. A UV curable solid ink is characterized by being a gel-like consistency at room temperature and a low viscosity liquid at an elevated temperature for jetting on a substrate. When the ejected ink hits the substrate, it changes phase from the liquid back to its more viscous gel consistency. Because these gel inks solidify rapidly upon cooling, these inks have advantages over water or solvent-based inks that may de-wet from many surfaces, such as plastics, cardboard, and the like. Once the gel ink is exposed to UV radiation, the ink is cured to form a cross-linked polymer matrix resulting in a very hard and durable mark on the substrate.
There remains a desire to level or spread the ink to reduce the so-called “corduroy” effect and achieve more uniform gloss, mask missing jets, and thinner layers required for applications such as packaging. Conventional leveling methods and devices, however, cannot be used with the gel inks due to their unique properties. Since these inks have a paste-like consistency, the inks have very little cohesive strength prior to curing. In addition, the inks are designed such that they have good affinity to many materials. As a result, conventional methods for flattening a layer of ink tend to fail when used with the UV gel ink because the ink will stick to the leveling device and undergo cohesive failure (i.e. splits) leaving residual ink behind on the leveling device. It is not possible to flatten the ink after it has cured either because the polymerized material is extremely tough and non-yielding.
Ampo, K. et al. “Leveling Viscous Fluids Using Ultrasonic Waves”, JJAPS, (2004), vol 43, pp 2857-2861 describes a method for leveling viscous fluids, in this case a photoresist material, on substrates using ultrasound techniques. In this paper, it has been proposed that such a method may be advantageous over traditional coating methods such as slit nozzle methods (due to unevenness and orientation dependence), and spin-coating (challenging for large substrates and wasteful, due to spilling off of the edges). The paper describes experiments done to level a photoresist material with a viscosity of 10 cps coated onto a non-porous substrate. However, this paper does not address how one could level a viscous gel ink (with viscosity of 106 cps). Moreover, there are no teachings in this publication on how one would level a viscous gel ink on a substrate such a paper.
U.S. Pat. No. 5,376,402 which is hereby incorporated by reference herein in its entirety, describes, in embodiments, an ultrasonically assisted coating method for applying a smooth layer of coating material on a surface of a moving web. In this patent, the coating material is deposited using a die onto a moving web, where the ultrasonic energy generator is applying ultrasonic energy in a variety of modes, directly to the web, to the die itself, and through the air. However, this patent only describes materials deposited by ‘contact’ approaches, such as slot-fed knife coating, roll-coating, and extrusion coating. Furthermore in the single example cited in the patent, the highest viscosity material tested was a solvent-based rubber coating with a viscosity of 5,000 cps, at a thickness of 63.5 um. Thus, this reference does not address how one would level a viscous gel ink on a substrate.
Thus, there exists a need for a non-contact approach to level the images printed with gelled UV inks prior to curing. The present embodiments are thus directed to curable gel inks, and more particularly, UV curable gel inks and methods specially adapted for leveling these inks on substrates.